The valve stems seals tend to be more useful on the intake valves, the minute vacuum produced (remember these are the intake valves so there is suction/vacuum on intake) keeps them down the bottom where they are most effective, keeping a decent seal. What little fumes escape through or past the exhaust valve stems/guides are expelled through the outlet at the top of the rocker cover, along with all the other fumes.

What you have is the umbrella sort which is basically a shroud. The better ones have a little spring collar that grips the valve guides, these are most effective when there is a groove cut into the valve guides to allow a decent connection. I will try and get some photo's if that'll be any help explaining things?